Temperature correction method and subsystem for automotive evaporative leak detection systems

ABSTRACT

A method and sensor or sensor subsystem permit improved evaporative leak detection in an automotive fuel system. The sensor or sensor subsystem computes temperature-compensated pressure values, thereby eliminating or reducing false positive or other adverse results triggered by temperature changes in the fuel tank. The temperature-compensated pressure measurement is then available for drawing an inference regarding the existence of a leak with reduced or eliminated false detection arising as a result of temperature fluctuations.

[0001] This application claims the benefit of the Oct. 2, 1997 filingdate of provisional application No. 60/060,858.

FIELD OF THE INVENTION

[0002] The present invention relates, in general, to automotive fuelleak detection methods and systems and, in particular, to a temperaturecorrection approach to automotive evaporative fuel leak detection.

BACKGROUND OF THE INVENTION

[0003] Automotive leak detection systems can use either positive ornegative pressure differentials, relative to atmosphere, to check for aleak. Pressure change over a given period of time is monitored andcorrection is made for pressure changes resulting from gasoline fuelvapor.

[0004] It has been established that the ability of a leak detectionsystem to successfully indicate a small leak in a large volume isdirectly dependent on the stability or conditioning of the tank and itscontents. Reliable leak detection can be achieved only when the systemis stable. The following conditions are required:

[0005] a) Uniform pressure throughout the system being leak-checked;

[0006] b) No fuel movement in the gas tank (which may results inpressure fluctuations); and.

[0007] c) No change in volume resulting from flexure of the gas tank orother factors.

[0008] Conditions a), b), and c) can be stabilized by holding the systembeing leak-checked at a fixed pressure level for a sufficient period oftime and measuring the decay in pressure from this level in order todetect a leak and establish its size.

SUMMARY OF THE INVENTION

[0009] The method and sensor or subsystem according to the presentinvention provide a solution to the problems outline above. Inparticular, an embodiment of one aspect of the present inventionprovides a method for making temperature-compensated pressure readingsin an automotive evaporative leak detection system having a tank with avapor pressure having a value that is known at a first point in time.According to this method, a first temperature of the vapor is measuredat substantially the first point in time and is again measured at asecond point in time. Then a temperature-compensated pressure iscomputed based on the pressure at the first point in time and the twotemperature measurements.

[0010] According to another aspect of the present invention, theresulting temperature-compensated pressure can be compared with apressure measured at the second point in time to provide a basis forinferring the existence of a leak.

[0011] An embodiment of another aspect of the present invention is asensor subsystem for use in an automotive evaporative leak detectionsystem in order to compensate for the effects on pressure measurement ofchanges in the temperature of the fuel tank vapor. The sensor subsystemincludes a pressure sensor in fluid communication with the fuel tankvapor, a temperature sensor in thermal contact with the fuel tank vapor,a processor in electrical communication with the pressure sensor andwith the temperature sensor and logic implemented by the processor forcomputing a temperature-compensated pressure based on pressure andtemperature measurements made by the pressure and temperature sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 shows, in schematic form, an automotive evaporative leakdetection system in the context of an automotive fuel system, theautomotive leak detection system including an embodiment of atemperature correction sensor or subsystem according to the presentinvention.

[0013]FIG. 2 shows, in flowchart form, an embodiment of a method fortemperature correction, according to the present invention, in anautomotive evaporative leak detection system.

DETAILED DESCRIPTION

[0014] We have discovered that, in addition to items a), b), and c) setforth in the Background section above, another condition that affectsthe stability of fuel tank contents and the accuracy of a leak detectionsystem is thermal upset of the vapor in the tank. If the temperature ofthe vapor in the gas tank above the fuel is stabilized (i.e., does notundergo a change), a more reliable leak detection test can be conducted.

[0015] Changes in gas tank vapor temperature prove less easy tostabilize than pressure. A vehicle can, for example, be refueled withwarmer than ambient fuel. A vacuum leak test performed after refuelingunder this condition would falsely indicate the existence of a leak. Thecool air in the gas tank would be heated by incoming fuel and cause thevacuum level to decay, making it appear as though there were adiminution of mass in the tank. A leak is likely to be falsely detectedany time heat is added to the fuel tank. If system pressure wereelevated in order to check for a leak under a positive pressure leaktest, and a pressure decay were then measured as an indicia of leakage,the measured leakage would be reduced because the vapor pressure wouldbe higher than it otherwise would. Moreover, measured pressure wouldalso decline as the vapor eventually cools back down to ambientpressure. A long stabilization period would be necessary to reach thestable conditions required for an accurate leak detection test.

[0016] The need for a long stabilization period as a precondition to anaccurate leak detection test result would be commerciallydisadvantageous. A disadvantageously long stabilization period can becompensated for and eliminated, according to the present invention, byconducting the leak detection test with appropriate temperaturecompensation even before the temperature of the vapor in the gas tankhas stabilized. More particularly, a detection approach according to thepresent invention uses a sensor or sensor subsystem that is able toeither:

[0017] 1) Provide information on the rate of change of temperature aswell as tank vapor pressure level, and correct or compensate for thechange in temperature relative to an earlier-measured temperaturereference; or

[0018] 2) Provide tank pressure level information corrected (e.g.,within the sensor to a constant temperature reference, the result beingavailable for comparison with other measured pressure to conduct aleak-detection test.

[0019] In order to obtain the data required for option 1), two separatevalues-must be determined (tank temperature rate of change and tankpressure) to carry out the leak detection test. These values can beobtained by two separate sensors in the tank, or a single sensorconfigured to provide both values.

[0020] Alternatively, if tank pressure is to be corrected in accordancewith option 2), then a single value is required. This single value canbe obtained by a new “Cp” sensor (compensated or corrected pressuresensor or sensor subsystem) configured to provide a corrected pressure.

[0021] To obtain this corrected pressure, P_(c), the reasonableassumption is made that the vapor in the tank obeys the ideal gas law,or:

PV=nRT

[0022] where:

[0023] P=pressure;

[0024] V=volume;

[0025] n=mass;

[0026] R=gas constant; and

[0027] T=temperature.

[0028] This expression demonstrates that the pressure of the vaportrapped in the tank will increase as the vapor warms, and decrease as itcools. This decay can be misinterpreted as leakage. The Cp sensor orsensor subsystem, according to the present invention, cancels the effectof a temperature change in the constant volume gas tank. To effectuatesuch cancellation, the pressure and temperature are measured at twopoints in time. Assuming zero or very small changes in n, given that thesystem is sealed, the ideal gas law can be expressed as:

P ₁ V ₁ /RT ₁ =P ₂ V ₂ /RT ₂

[0029] Since volume, V, and gas constant, R, are reasonably assumed tobe constant, this expression can be rewritten as:

P ₂ =P ₁(T ₂ /T ₁).

[0030] This relation implies that pressure will increase from P₁ to P₂if the temperature increases from T₁ to T₂ in the sealed system.

[0031] To express this temperature-compensated or -corrected pressure,the final output, P_(c), of the Cp sensor or sensor subsystem will be:

P _(c) =P ₁−(P ₂ −P ₁)

[0032] where P_(c) is the corrected pressure output. Substituting forP₂, we obtain:

P _(c) =P ₁−(P ₁(T ₂ /T ₁)−P ₁).

[0033] More simply, P_(c) can be rewritten as follows:

P _(c) =P ₁(2−T ₂ /T ₁).

[0034] As an example using a positive pressure test using the Cp sensoror sensor subsystem to generate a temperature-compensated or -correctedpressure output, the measured pressure decay determined by a comparisonbetween P_(c) and P₂ (the pressure measured at the second point in time)will be a function only of system leakage. If thetemperature-compensated or -corrected pressure, P_(c), is greater thanthe actual, nominal pressure measured at the second point in time (i.e.,when T₂ was measured), then there must have been detectable leakage fromthe system. If Pc is not greater than the nominal pressure measured atT₂, no leak is detected. The leak detection system employing a sensor orsubsystem according to the present invention will reach an accurateresult more quickly than a conventional system, since time will not bewasted waiting for the system to stabilize. The Cp sensor or subsystemallows for leakage measurement to take place in what was previouslyconsidered an unstable system.

[0035]FIG. 1 shows an automotive evaporative leak detection system(vacuum) using a tank pressure sensor 120 that is able to provide thevalues required for leak detection in accordance with options 1) and 2)above. The tank pressure/temperature sensor 120 should be directlymounted onto the gas tank 110, or integrated into the rollover valve 112mounted on the tank 110.

[0036] Gas tank 110, as depicted in FIG. 1, is coupled in fluidcommunication to charcoal canister 114 and to the normally closedcanister purge valve 115. The charcoal canister 114 is in communicationvia the normally open canister vent solenoid valve 116 to filter 117.The normally closed canister purge valve 115 is coupled to manifold(intake) 118. The illustrated embodiment of the sensor or subsystem 120according to the present invention incorporates a pressure sensor,temperature sensor and processor, memory and clock, such components allbeing selectable from suitable, commercially available products. Thepressure and temperature sensors are coupled to the processor such thatthe processor can read their output values. The processor can eitherinclude the necessary memory or clock or be coupled to suitable circuitsthat implement those functions. The output of the sensor, in the form ofa temperature-compensated pressure value, as well as the nominalpressure (i.e., P₂), are transmitted to processor 122, where a check ismade to determine whether a leak has occurred. That comparison,alternatively, could be made by the processor in sensor 120.

[0037] In an alternative embodiment of the present invention, the sensoror subsystem 120 includes pressure and temperature sensing deviceselectronically coupled to a separate processor 122 to which is alsocoupled (or which itself includes) memory and a clock. Both this and thepreviously described embodiments are functionally equivalent in terms ofproviding a temperature-compensated pressure reading and a nominalpressure reading, which can be compared, and which comparison cansupport an inference as to whether or not a leak condition exists.

[0038]FIG. 2 provides a flowchart 200 setting forth steps in anembodiment of the method according to the present invention. These stepscan be implemented by any processor suitable for use in automotiveevaporative leak detection systems, provided that the processor: (1)have or have access to a timer or clock; (2) be configured to receiveand process signals emanating, either directly or indirectly from a fuelvapor pressure sensor; (3) be configured to receive and process signalsemanating either directly or indirectly from a fuel vapor temperaturesensor; (4) be configured to send signals to activate a pump forincreasing the pressure of the fuel vapor; (5) have, or have access tomemory for retrievably storing logic for implementing the steps of themethod according to the present invention; and (6) have, or have accessto, memory for retrievably storing all data associated with carrying outthe steps of the method according to the present invention.

[0039] After initiation, at step 202 (during which any requiredinitialization may occur), the processor directs pump 119 at step 204,to run until the pressure sensed by the pressure sensor equals apreselected target pressure P₁. (Alternatively, to conduct a vacuum leakdetection test, the processor would direct the system to evacuate to anegative pressure via actuation of normally closed canister purge valve115). The processor therefore should sample the pressure reading withsufficient frequency such that it can turn off the pump 119 (or closevalve 115) before the target pressure P₁ has been significantlyexceeded.

[0040] At step 206, which should occur very close in time to step 204,the processor samples, and in the memory records, the fuel vaportemperature signal, T₁, generated by the temperature sensor. Theprocessor, at step 208, then waits a preselected period of time (e.g.,between 10 and 30 seconds). When the desired amount of time has elapsed,the processor, at step 210, samples and records in memory the fuel vaportemperature signal, T₂, as well as fuel vapor pressure, P₂.

[0041] The processor, at step 212, then computes an estimatedtemperature-compensated or corrected pressure, P_(c), compensating forthe contribution to the pressure change from P₁ to P₂ attributable toany temperature change (T₂−T₁).

[0042] In an embodiment of the present invention, thetemperature-compensated or corrected pressure, P_(c), is computedaccording to the relation:

P _(c) =P ₁(2−T ₂ /T ₁)

[0043] and the result is stored in memory. Finally, at step 214, thetemperature-compensated pressure, P_(c), is compared by the processorwith the nominal pressure P₂. If P₂ is less than P_(c), then fuel musthave escaped-from the tank, indicating a leak, 216. If, on the otherhand, P₂ is not less than P_(c), then there is no basis for concludingthat a leak has been detected, 218.

[0044] The foregoing description has set forth how the objects of thepresent invention can be fully and effectively accomplished. Theembodiments shown and described for purposes of illustrating thestructural and functional principles of the present invention, as wellas illustrating the methods of employing the preferred embodiments, aresubject to change without departing from such principles. Therefore,this invention includes all modifications encompassed within the spiritof the following claims.

1-16 (cancelled)
 17. A method of diagnosing an evaporative emissioncontrol system to determine if a leak is present in the system, themethod comprising: sealing the system from external influences;monitoring a pressure level within the system over a cooling period; andindicating a potential leak condition through a comparison of thepressure level within the system and a given threshold.
 18. The methodaccording to claim 17, wherein monitoring comprises monitoring thepressure level to determine an initial pressure at a commencement of thecooling period and a second pressure at the end of the cooling period.19. The method according to claim 18, wherein indicating comprisesindicating the potential leak condition through the comparison of thesecond pressure and the given threshold.
 20. The method according toclaim 19, wherein the given threshold is determined from the initialpressure.
 21. The method according to claim 20, wherein the giventhreshold comprises a temperature-compensated initial pressure.
 22. Themethod according to claim 17 wherein monitoring comprises providing atemperature-compensated pressure sensor having a pressure sensingelement and a temperature sensing element.
 23. The method according toclaim 22 wherein monitoring further comprises: coupling a processor tothe pressure sensing element and to the temperature sensing element; andreceiving, respectively, pressure and temperature signals therefrom. 24.The method according to claim 23 wherein monitoring further comprises:implementing logic by the processor for computing atemperature-compensated pressure on the basis of pressure andtemperature measurements.
 25. The method according to claim 24 furthercomprising: computing the temperature-compensated pressure as a functionof the pressure at a first point in time and the temperature measured atsubstantially the first point, and at a second point, in time.
 26. Themethod according to claim 25 wherein the function comprises theexpression: P _(C) =P ₁(2−T ₂ /T ₁) where P_(C) is thetemperature-compensated pressure, P₁ is the pressure measured at thefirst point in time, T₁ is the temperature measured at substantially thefirst point in time, and T₂ is the temperature measured at the secondpoint in time.
 27. The method according to claim 17 wherein monitoringcomprises providing a sensor subsystem for compensating for the effectson pressure measurement of changes in the temperature of the systemvapor.
 28. The method according to claim 27 wherein the subsystemcomprises a pressure sensor in fluid communication with the system vaporand a temperature sensor in thermal contact with the system vapor. 29.The method according to claim 28 wherein monitoring further comprises:providing a processor in electrical communication with the pressuresensor and with the temperature sensor; and implementing logic by theprocessor for computing a temperature-compensated pressure based onpressure and temperature measurements made by the pressure andtemperature sensors.
 30. The method according to claim 29 wherein theimplementing logic comprises computing the temperature-compensatedpressures as a function of pressure measured at a first point in timeand of the temperature measured at the first, and at a second, point intime.
 31. The method according to claim 30 wherein the functioncomprises: P _(C) =P ₁(2−T ₂ /T ₁) where P_(C) is thetemperature-compensated pressure, P₁ is the pressure measured at thefirst point in time, T₁ is the temperature measured at substantially thefirst point in time and T₂ is the temperature measured at a second pointin time.
 32. The method according to claim 31 further comprising:indicating the potential leak condition through a comparison of thetemperature-compensated pressure, P_(C) and the pressure measured at thesecond point in time, P₂.
 33. The method according to claim 32, whereinthe leak condition is determined to exist if the pressure P₂ is lessthan the temperature-compensated pressure, P_(C).